Question

I am just continuously hitting a wall with this one, I cannot solve it. I am trying to get a regex that executes as:

(axb|cxd)

except without repeating x in the expression (as it is really long encoding-matching expression). This is tested against a large string where I only need to match x for string replacement, so zero-width lookahead/behind is an option. Any ideas?

a,b,c, and d are reasonably small, so they can repeat if it makes the expression easier to form.

Thanks in advance.

Answer 1

If you really don’t want to repeat x in the regular expression, then you’ll have to apply some manual search logic. You can search for x with the regular expression and then, based on where it was found, look for a before it and b after it or c before it and d after it. But, that would likely be more work than just repeating it in the regular expression as in Gabe’s answer.

For example, you could do this:

var re = /(a|c)x(b|d)/;
var matches = str.match(re);
if (matches) {
    // exclude axd or cxb
    if ((matches[1] + matches[2]).match(/ab|cd/)) {
        // found axb or cxd
    }
}

But, honestly, it seems like it would just be easier to include x twice and let the regex engine do the dirty work for you:

if (str.match(axb|cxd)) {
    // found match
}

Or, if it’s too cumbersome to put x in the string twice, then build the regex yourself using javascript string math:

var x = "long complicated regex";
var re = new RegExp("a" + x + "b|c" + x + "d");
if (str.match(re)) {
    // found match
}